Design and Manufacture of Testing Equipment for Switchgear

Design and Manufacture of Testing Equipment for Switchgear

By Terry Cousins and Luis Valentim, TLC Engineering Solutions

Abstract

Switchgear is a key component in electrical power systems. It is essential to verify performance after design,

manufacture or repair. This paper will discuss a range of automated testing equipment that are used to test

switchgear. The test equipment has been designed and manufactured in South Africa in collaboration with

switchgear manufacturers and testing authorities. Automated analysis of the measurements as well as archiving of

results for future recall and comparison will be presented.

Introduction

The definition of switchgear includes electrical disconnects, fuses and circuit breakers used to isolate electrical

equipment. Circuit breakers are mechanical devices connected to the electrical system, where they must provide a

suitable path for the flow of the electric current as well as provide protection and control of the electric circuit by

either initiating or stopping the current flow. Circuit breakers design and performance verification requires specific

tests to be performed. This paper will present a range of testing systems for circuit breakers from the instrumentation

and data acquisition perspective.

Circuit Breaker Testing

Circuit breakers are designed and manufactured to operate correctly under a range of conditions. These include

switching currents up to the rated load, as well as interrupting overload or fault currents several times the rated

current. In the event of a malfunction, the circuit breaker as well as parts of the electric system may sustain severe

damage. It is therefore essential to have a high degree of confidence in the performance of any circuit breakers. This

confidence level can only be attained by years of operating experience, or by extensive testing under conditions that

simulate those that are encountered in the field applications.

Testing will usually be performed to ensure compliance with a local or international standard. Special tests may also

be performed by manufacturers to evaluate new designs. The tests that are common to most circuit breakers include:

Mechanical Operation

o Performance of the mechanism to open and close contacts

o Endurance of the mechanism

Electrical Operation

o Ability to operate safely with rated loads

o Ability to safely isolate overloads and short circuits

Mechanical Testing

The most basic function of any circuit breaker is to open and close the contacts on command. According to a CIGRE

report [1] more than 90% of circuit breaker failures are attributed to mechanical causes. These findings confirm the

fact that circuit breakers are primarily mechanical devices that are called upon to perform an electric function. Most

of the time circuit breakers remain closed and simply act as electrical conductors. When the breaker performs a

protective function, from the combined electrical and mechanical point of view, the contact structure is probably the

most essential and critical component. A second and equally important component is the operating mechanism

employed to produce the motion of the contacts.

A circuit breaker is designed to interrupt a short circuit current. The fault energy is proportional to the square of the

fault current times the duration of the fault. The breaker should clear the fault in the minimum time to limit fault

damage. This requires that the breaker must operate at a speed greater than a specified minimum speed determined

by the design. The current must also be interrupted in such a way that the arc does not re-strike. The interrupting

time or breaking time of a circuit breaker has a common definition according to ANSI and IEC. It consists of the

time, from the instant the trip coil is energized to the instant of complete interruption of the current flow through the

circuit breaker (taking into account the maximum arcing time required for interruption). Note that the contact parting

time, as defined by ANSI, is the summation of the relay time plus the opening time. These relationships are

illustrated in Fig. 1 below [2].

Fig 1: Interrupting Time Relationships

The opening and closing velocities, as well as stroke, or travel distance, are the most important operating

characteristics of a circuit breaker. They are dictated primarily by the requirements imposed by the contacts.

Opening and closing velocities are important to the contacts in order to avoid contact erosion as well as contact

welding. Since contact stroke is synonymous with contact gap, circuit breaker stroke is primarily related to the

ability of the circuit breaker to withstand the required operating dielectric stresses.

The opening and closing motion curve of a circuit breaker is shown in Fig. 2 [3].

Fig. 2: Circuit Breaker Open and Closing Motion

Open / Close Speed Test

This test is performed to confirm the key mechanical parameters of the switchgear. This includes the open and close

speeds, travel distance and contact closing and parting times. Equipment for performing this test consists of:

Computer equipment with application software for performing the test and analysing the results

Displacement transducer for measuring the travel of the circuit breaker

Continuity measurement for measuring the contact open and closing

Control circuit and power supply for energising the trip and close coils

Current clamps to measure trip and close coil currents

The equipment is shown below in Fig. 3.

Fig. 3: Test Equipment for Open / Close Speed Test

The test system also requires a power supply to energise the close and trip coils as well as a supply for charging the

mechanism.

The travel measurement is obtained by connecting a linear displacement transducer to the contact mechanism. The

output of this transducer is then calibrated to correspond with the distance moved by the contacts.

A typical graphic display from the test is shown in Fig. 4.

Fig. 4: Open / Close Speed Trace

The upper trace is the travel. The centre three traces are the continuity of the contacts. This example shows severe

contact bounce when the contacts close. These results are then automatically analysed by the computer software and

a report is produced similar to the report in Fig. 5.

Fig. 5: Open / Close Speed Test Analysis Results

These results can be printed and are saved to a database for future reference.

Short Circuit Testing

Short circuit testing is one of the most essential tasks that must to be performed on equipment to verify equipment

suitability. It is also an essential tool from the design point of view, as much design knowledge is based on

experimental findings and consequently testing becomes the tool that is used for the development of circuit breakers.

Short circuit testing presents a challenge, because what must be replicated is the interaction of a mechanical device,

the circuit breaker, and the electric system where the switching conditions can vary quite widely depending upon the

system configuration and so can the conditions which must be demonstrated by tests.

Another challenge has always been the development of appropriate test methods that can overcome the potential

lack of sufficient available power at the test facility. One has to consider that the test laboratory should basically be

able to supply the same short circuit capacity as that of the system for which the circuit breaker that is being tested is

designed. However, this is not always possible, especially at the upper end of the power ratings. A typical test circuit

is shown in Fig. 6 below [2].

Fig. 6: Elementary Schematic of Short Circuit Test

The test circuit consists of a power source (G) which can be either a specially designed short circuit generator or the

system's electric network. For the protection of the supply, a high capacity back-up circuit breaker (BUB) is used for

interrupting the test current in the event that the circuit breaker being tested (TB) would fail to interrupt the current.

In series with the back-up circuit breaker there is a high speed making switch (MS) which is normally a

synchronized switch capable of independent pole operation and of precise control for closing the contacts at a

specific point on the current wave. Current limiting reactors (L) are connected in series with the make switch. These

are used to limit the magnitude of the test current to its required value. Specially designed test transformers (T) are

connected between the test circuit breaker and the power source. These transformers are used primarily to allow for

flexibility for testing at different voltage levels, and in addition to provide isolation between the test generator and

the device that is being tested. Across the test breaker terminals a bank of transient recovery voltage shaping

capacitors (C) is connected. To measure the voltages at least one set of voltage dividers (V) are used. The short

circuit current flowing through the test device is measured by means of a shunt (Is). Current transformers can be

used especially for the current at the upstream side of the circuit breaker.

The test equipment (instrumentation) supplied for the short circuit testing consists of:

High Speed Digitiser

o Measures voltage and current waveforms up to 5 MS/s with 12 bit resolution

o Capture up to 4 million samples

Precision Timer

o Used to synchronise all the test events to a precise timebase

Computer for Data Analysis and Presentation

A precision timer was specially designed and engineered for this application [4]. A programmable logic controller

(PLC) is used in the test facility to provide the necessary control and safety interlocks. It does not however have a

sufficiently accurate timebase to provide for the synchronization of the make switch.

Under short circuit conditions the current flowing through the circuit breaker can exceed 100kA and equipment is

highly sensitive to the time taken to clear the fault. In order to test circuit breakers it is necessary to provide an exact

fault current to ensure the device is not destroyed by the test. The fault current is obtained by connecting the breaker

to a known load and switching on and off at a specific point of the 50Hz ac wave. This is illustrated in Fig. 7.

Fig. 7: Precision Point-on-Wave Switching with Precision Timer

The Precision Timer requirements were as follows:

10 microseconds accuracy in a 20 second timeframe

24 timer channels

Independent pulse duration for each channel

Absolute or point-on-wave time synchronization

Graphical entry of timer parameters

The actual design achieves timing accuracies around 2µS. The block diagram of the Precision Timer is shown in

Fig. 8

Fig. 8: Precision Timer Block Diagram

A graphical user interface was developed to facilitate entry and display of the test sequencing times. This is shown

in Fig. 9.

Fig. 9: Precision Timer Test Sequencing

The waveforms from the short circuit test were acquired by the high speed digitizer and displayed on the computer

screen in the control room. For some tests this had to be completed in a minimum time to ensure subsequent tests

were performed according to a specific schedule. A typical test waveform is shown in Fig. 10.

Fig. 10: Short Circuit Test Sample Waveform

A number of automated analysis features are available by selecting an icon on the screen. These include:

Relative Measurements

Markers

STL Peak calculation (STL parameters are specified by the Short-Circuit Testing Liaison Agreement (STLA).

This is an Agreement Group of International Certification Bodies concerning the type testing of high voltage electrical

power equipment.)

STL RMS calculation

I2t Integration

AC RMS calculation

Removal of Mean

Export to comma separated text (CSV) Format

All the raw data and results can be stored in a database for future reference. A typical three phase calibration test

waveform is shown in Fig. 11.

Fig. 11: Example Three Phase Calibration Waveform

In addition to the short circuit test method described above where fault currents are interrupted, there are several

other test methods which involve testing at a specific current to determine the current carrying capacity of the

breaker.

Earth Leakage Circuit Breaker Testing

The earth leakage circuit breaker (ELCB) is primarily used for the protection of human life. It has been found from

numerous studies that once they have been introduced, they dramatically decrease the number of fatalities from

electric shock [5]. The use of ELCB in buildings is required by law in South Africa (SANS 10142-1: 2003 - Code of

practice for the wiring of premises) [6]. The performance of the ELCB is regulated by local and international

standards e.g.

SANS 767-1 (SABS 767-1), Earth leakage protection units – Part 1: Fixed earth leakage protection circuit-

breakers.

IEC 1008-1 - Residual Current Operated Circuit Breakers

UL 943 - Ground Fault Circuit Interrupters

Manufacturers of ELCB need to provide proof of compliance by submitting circuit breakers for testing to testing

laboratories.

An ELCB Test Unit (instrumentation) was developed for this compliance testing. It consists of:

Computer based test sequencer

Digitiser

Variable Current Source

Control Switching

Tests basically require the injection of a leakage current into one of the phases. A simplified ELCB test circuit is

shown in Fig. 12.

Fig. 12: Earth Leakage Test Unit Block Diagram

The user interface for the test setup is designed so that any one of the required tests can be performed. All of the

results are stored in a database for future reference. A typical test setup screen is shown in Fig 13. The test follows

the format of the standards requirement so that all the required tests must be completed on the sample batch.

Fig. 13: Earth Leakage Test Unit Operator Interface

The equipment is housed in a 19” instrumentation panel as shown in Fig. 14.

Fig. 14: Earth Leakage Test Unit Equipment

Endurance Testing

A useful test to perform on switchgear is a rated load test for an extended period. This is used to determine if any

equipment malfunctions and can also be used to evaluate thermal information.

A constant current test rig was developed for this test with the following specifications:

Used to test circuit breakers up to 400A

Programmable constant voltage / current

Closed loop control to keep current at required value

Detect if any devices trip

Record results

The testing system consists of a 3.2kW power supply and adjustable transformer which provides the test current.

The output is set by a computer system which ensures the output remains within set limits. The computer also saves

the measured value and saves the test results to a database. While a temperature recording device did not form part

of this test rig, it could be included if required. The constant current supply user interface and results are shown in

Fig 15.

Fig. 15: Endurance Test Setup Screen

Fig. 16: Final Current Adjustment

Once the test parameters have been entered, the operator selects the required current and adjusts the variable

transformer until the test current is in the green band as displayed in Fig. 16. The computer then regulates the current

in a closed loop control until the test is complete. The test can conclude either by successfully reaching the specified

end period, or one of the test devices can trip which will result in the failure of the test batch. All results from the

test are saved to a test database that can be used for record keeping and results analysis. A sample database screen is

shown in Fig. 17.

Fig. 17: Test Results Database

The test computer equipment is housed in a modular 19” rack mount system shown in Fig. 18.

Fig. 18: Endurance Test Panel

Conclusion

A number of diverse switchgear testing systems have been successfully developed and deployed locally for

switchgear manufacturers and testing laboratories. The test equipment design requires close collaboration between

switchgear manufacturer, testing laboratory and test equipment supplier. The equipment requirements and standards

change over time. The test equipment must be designed to be flexible so as to accommodate future enhancements.

References

[1] CIGRE SC13, High Voltage Circuit Breaker Reliability Data for Use in System Reliability Studies, CIGRE

Publication, Paris, France 1991.

[2] Ruben D. Garzon: High Voltage Circuit Breakers, Design and Applications, 2nd Ed, Marcel Dekker Inc.,

New York, 2002

[3] TM1800 Circuit Breaker Analyzer System User Manual, GE Energy

[4] T Cousins: Development of a Precision Timing System for Circuit Breaker Testing, Quantum, November

1999

[5] Viv Cohen: Electrical Shock And Fire Hazard Protection – Conquering The Limitations, Elektron,

September 1993.

[6] SANS 10142-1:2003 (Ed. 1.1) The wiring of premises Part 1: Low-voltage installations

Contact details

For further information, contact Terry Cousins, Director, TLC Engineering Solutions on 011 463 3860 or

[email protected]